Can't expand a partition

I'm running a Mac Mini with Windows 8 on it, and I've got an external hard drive which stores steam games etc. I've found out that I use Windows 99% of the time, and therefore don't need the partition allocated to OSX anymore. However, I can't expand the partition in disk management, as the extend/shrink buttons are grayed out. Keep in mind that the partition is primary, and it's healthy as well.

Thanks.